The utilization of a helicopter frequently involves a hover maneuver in which the helicopter is maintained at a fixed point with respect to the earth and at a low altitude. Controlling the aircraft during a hover is very demanding on the pilot, particularly in gusty wind conditions or tight operating conditions. This workload demand causes fatigue and prevents the pilot from performing any other duties.
Automatic hover position control systems were introduced some time ago in an attempt to reduce pilot workload and fatigue during hovering maneuvers. An example of an existing hover hold system is disclosed in commonly owned U.S. Pat. No. 4,213,584 (Tefft et al). Prior systems, such as Tefft, employ a number of sensor systems, some of which include Doppler radar velocity systems, TACNAV or Global Position Sensing (GPS) systems, accelerometers and wind measuring systems. Unfortunately, prior automatic hover control systems either incorporate a combination of sensor systems which provide inadequate data or they do not utilize the sensor systems they incorporate effectively and, therefore, suffer from a number of deficiencies which severely limit their performance.
One such deficiency is that prior systems do not perform well in either steady wind or gusty conditions because they do not sense wind with the purpose of anticipating its effects on the aircraft. Instead, existing controllers must wait for aircraft velocity and/or integrated velocity errors to develop before compensating for the wind. The subsequent lag in response time causes large position errors and poor hover position hold performance.
Also, inefficiencies in the previously mentioned sensor systems cause prior automatic hover hold control systems to suffer from drift and steady state errors. For instance, prior systems that rely exclusively on Doppler radar systems to measure ground speed experience null shifts and low frequency errors because Doppler systems are inefficient at low aircraft velocities. This adversely affects overall system performance.
Yet another deficiency in prior automatic hover hold control systems arises when aggressive turning maneuvers are performed by the pilot. If the aircraft yaws between 180 to 360 degrees, the integral of ground-plane referenced velocity (which is used for a pseudo-position signal because of the aforementioned TACNAV deficiencies) may go to zero even though a steady inertial position error may be accumulating.
All of the aforementioned deficiencies require pilot intervention, thereby distracting him or her from other duties and causing fatigue. An improved system eliminating these disadvantages is highly desirable.